A dynamic scattering liquid crystal cell comprises a thin layer of a dynamic scattering liquid crystal composition between two electrodes. Dynamic scattering liquid crystal compositions comprise a liquid crystal material having negative dielectric anisotropy containing both an ionic dopant to carry the charge through the cell and an aligning agent to insure uniform alignment of the liquid crystal molecules with respect to the electrodes. The aligning agent and the ionic dopant are generally added to the liquid crystal composition prior to filling the cell.
However, when the cell walls are made of inexpensive soda-lime glass coated with a transparent conductive layer, e.g. of tin oxide or indium oxide, the surface of the glass adsorbs the ionic dopant and the aligning agent at a faster rate than the liquid crystal material and thus the cell becomes nonuniform in composition throughout the cell.
It is known that in filling the cell through a single fill port, much of the ionic dopant and the aligning agent are adsorbed by the glass and consequently the liquid crystal material at the far end of the cell from the port contains little or none of these additives. Thus, when a voltage is applied to the cell, portions of the cell appear nonuniform, hazy, or even fail to dynamic scatter at all. The larger the liquid crystal cell, the more pronounced this effect becomes.
One method suggested in the past to overcome this problem is to allow two ports in the cell wall seal, and to flush the cell with a large amount of the desired liquid crystal composition so that the cell walls become saturated with the additive. Then, when a fresh batch of the liquid crystal composition is added to fill the cell, little or no additional stripping of the additives occurs and the liquid crystal composition remains uniform throughout the cell. This method has the disadvantage that a large quantity of liquid crystal composition is lost during the flushing, and two ports must be sealed in the cell walls.
In another method, a gaseous medium carrying the additive material is flushed through the cell prior to filling the cell with the liquid crystal composition. This method also precoats the cell walls, but again requires two fill ports. These methods are further described in U.S. Pat. No. 3,698,449.
Present commercial liquid crystal cells are sealed with a glass frit seal which has a single port for filling the cell. Sealing of this fill port to ensure an absolutely hermetic seal involves several steps of sputtering the walls of the fill port with metal layers and soldering over the hole. This is an expensive and essential step to preclude any liquid crystal material from leaking out of the cell, or, even more difficult, to preclude any air or moisture from entering the cell and degrading the liquid crystal material. Thus, to have to make two ports for filling dynamic scattering cells, rather than one, greatly adds to the expense of manufacture.
The cell walls cannot be coated with the dopant before the glass plates are sealed together, since they must be heated to about 500.degree. C. to melt the glass frit seal joining the plates. The organic dopants would be destroyed during this sealing step.
Thus it would be desirable to be able to fill dynamic scattering liquid crystal cells using a single fill port but avoiding any inhomogeneities in the composition of the liquid crystal within the cell which would cause molecular misalignment or failure to dynamic scatter.